Media captionThe BBC's John Sudworth reports from outside the court in Hefei

The first day of the trial of the wife of former high-flying Chinese lawmaker Bo Xilai on charges of murdering UK businessman Neil Heywood has ended.

Gu Kailai is accused of poisoning Mr Heywood in 2011 in Chongqing, where her husband was the Communist party head.

State media has called the case against her and an aide "substantial".

The country is preparing to install a new generation of leaders, and Bo Xilai had once been seen as a strong contender for one of the top jobs.

He was sacked in March and is currently under investigation for unspecified "disciplinary violations".

The BBC's John Sudworth says some Chinese leaders are said to welcome the demise of such an openly ambitious colleague, but the case still needs careful handling for fear it might taint the Communist Party itself.

'Irrefutable evidence'

Ms Gu, 53, who is a well-known lawyer, is being tried in the city of Hefei. It is not clear how long her trial might last or when any verdict might be delivered.

At the scene

By John SudworthBBC News, Hefei

Appropriately perhaps for a case that has engulfed China in a storm of scandal, a typhoon has blown in for the start of the trial.

Two British diplomats, greeted by a scrum of rain-soaked foreign journalists, are the only outside observers allowed access. And while the media is shut out, and given no glimpse of Gu Kailai, the authorities appear to be tolerating our presence. Even more unusually we are told a press conference will be held once the trial ends.

On the face of it, the case appears to be a simple if tawdry tale. Gu Kailai stands accused of killing Neil Heywood with poison but it is her marriage to Bo Xilai - one of China's 25 most senior politicians - that makes this case so explosive.

Initially the murder appears to have been covered up, raising serious questions about corruption and abuse of power at the heart of China's ruling elite. As always in China, what goes on in court will be as much about politics as justice. Gu Kailai may well be guilty, but her trial is a clear indication of two things: that the Communist Party now wants a speedy end to the case and her conviction suits its interests.

What they do with her husband, now suspended from his party posts, may be a more difficult conundrum.

Dozens of uniformed and plain-clothes police were stationed around the court building as it got under way.

British diplomats are being allowed to witness the trial but journalists will not be attending. Ms Gu is being represented by state-appointed lawyers.

Mr Heywood's body was found at a hotel in Chongqing in November 2011, and the death was recorded as a heart attack at the time.

But four months later Mr Bo's right-hand man, police chief Wang Lijun, fled to a US consulate to allege murder and a massive cover-up.

Ms Gu and her aide Zhang Xiaojun are now accused of killing Mr Heywood, who is said to have been a business associate.

State media said Ms Gu and her son Bo Guagua fell out with Mr Heywood over "economic interests" and that Ms Gu was worried about "Neil Heywood's threat to her son's personal security".

"The facts of the two defendants' crime are clear, and the evidence is irrefutable and substantial," a Xinhua news agency report said.

Ms Gu and her husband have not been seen in public since April, when the investigation was announced.

Bo Guagua, 24, is believed to be in the US after graduating from Harvard University.

"As I was cited as a motivating factor for the crimes accused of my mother, I have already submitted my witness statement," he wrote in an email to US broadcaster CNN on Wednesday.

"I hope that my mother will have the opportunity to review them," he wrote. "I have faith that facts will speak for themselves."

'Divided'

Discussion of the case has been very limited in Chinese media. In the week leading up to the trial, no reports have been observed in state press.

Comment also appears to be tightly controlled on the internet, with an increasing number of keywords related to the case apparently blocked.

"I have noticed that in China's weibo (Twitter-like microblogging sites) and the internet, there are people expressing the view that she should be given a fair trial," Lijia Zhang, a Beijing-based writer and journalist, told the BBC.

"I have to tell you that she's not a very popular figure here. But some people do believe she's the victim of a political struggle among the very top leaders."

Seven members of the politburo Standing Committee are due to retire later this year. Bo Xilai, now sacked from his official positions, had been tipped for the top until his fall from grace.

Bo Zhiyue, of the National University of Singapore, said China's leaders were keen to make the focus of the case criminal, rather than political.

''Bo Xilai is a controversial figure. The central leadership may be divided over how to handle Bo Xilai. I think they have some consensus over how to deal with Gu Kailai,'' he said.

He added that there were signs she would be treated with a degree of leniency, pointing to the suggestion in state media that Ms Gu was in some way trying to protect her son.

Jin Xiaopeng, a Beijing-based lawyer, told the BBC he believed that "due to Ms Gu's special status, the most she will get is a suspended death sentence".